Centralise DSH configuration values

[cptanalatriste]

✅ Checklist

• I have searched open and closed issues for duplicates.
• This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• The feature is still missing in the latest version.
• I have read through the documentation.
• This isn't an open-ended question (open a discussion if it is).

🍓 Suggested change

There are some DSH configuration values that are spread over multiple files, instead of being centralised in a single component. For example, the caddy container image, name, CPU requirements, memory requirements, and mount path have the same values on the modules gitea_server.py, hedgedoc_server.py, remote_desktop.py, and software_repositories.py.

Changing any of this values ----let's say, for example, that we discover Caddy requires more memory than 0.5GB-- would require editing all these files. We request to centralise these values in a single component (maybe a constants.py module)? To ease maintenance.

🚂 How could this be done?

We can have a central constants.py module, or multiple modules per concern (like caddy_constants.py).

[JimMadge]

Is this about configuration (things the user can set) or constants (things we have hard coded)?

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Update and clarification. I think if these are constants, it is sensible to keep them in the file where they are used, or if there are used in a few places in that module.
I think having a top level constants module is more confusing (and risks name clashes).

[jemrobinson]

To be honest, I would prefer to hard-code as much as possible. When you start using a new tool, it can be intimidating if there are hundreds of available configuration options and you're not sure what they all do. There are plenty of places where saying "we think the best value is X" is good enough for 99% of people, and the 1% can contribute a PR to make this user-configurable if they really need to.